1. Field of the Invention
This invention relates to a method of and an apparatus for controlling a thermal head, for instance, in a heat-sensitive plate making apparatus, and more particularly to a method of and an apparatus for controlling heater elements in such a thermal head.
2. Description of the Related Art
There has been known a heat-sensitive plate making apparatus which makes a plate (e.g., a stencil) by thermally perforating a plate material by selectively energizing some of heater elements of a thermal head arranged in a main scanning direction while moving the plate material in a sub-scanning direction relatively to the thermal head. In such a plate making apparatus, heat history control, in which the time for which each heater elements is energized is controlled according to its heat history, is carried out. For example, when a square area of the plate material is to be perforated in order to print a square such as shown in FIG. 21A, imperforation can occur at an upper edge portion 72 and left and right side edge portions 71 of the square area as shown in FIG. 21B if the heater elements of the thermal head corresponding to the square area is uniformly energized without performing the heat history control. This is probably because the heater elements corresponding to the left and right side edge portions 71 cannot be sufficiently heated since heater elements on the outer sides of the left and right side edge portions 71 are not at an elevated temperature and because the heater elements corresponding to the upper edge portion 72 are not heated at the preceding perforating timing. On the other hand, if all the heater elements corresponding to the square area are energized for a longer time so that even the heater elements corresponding to the upper edge portion 72 and the left and right edge portions 71 are heated to a temperature sufficient to perforate the plate material, the perforations corresponding to the area 73 in FIG. 21B will become too large in size though imperforation at the edge portions 71 and 72 can be prevented.
When the heat history control is performed so that when a certain pixel (will be referred to as xe2x80x9ca pixel of current interestxe2x80x9d, hereinbelow) is a black spot which involves perforation of the plate material and at least one of the pixels adjacent to the pixel of current interest on opposite sides thereof and a pixel immediately preceding to the pixel of current interest is a white spot which does not involve perforation of the plate material, the heater element corresponding to the pixel of current interest is energized for a longer time, the heater elements corresponding to the edge portions 71 and 72 can be heated to a temperature equivalent to that of the heater elements corresponding to the area 73, whereby the perforations in the square area can be uniform in size as shown in FIG. 21C.
However such a heat history control is disadvantageous in that particular heater elements are repeatedly energized for a longer time than the other heater elements and the particular heater elements deteriorate in durability. For example, the heater elements corresponding to the edge portions 71 and 72 are energized for a longer time than the other heater elements during several perforations, and when a plurality of perforations arranged in a row in the sub-scanning direction are formed to print a thin line, one of the heater elements is continuously energized for a longer time. Durability of particular heater elements deteriorates also in a heat history control where such particular heater elements are not energized for a longer time but are applied with a larger power per unit time. In other words, when power applied to particular heater elements is increased in order to prevent defective perforation due to fluctuation in heating temperature of the heater elements (the temperature to which the heater element is heated when energized), there is a fear that the thermal head can deteriorate in durability.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and an apparatus for controlling a thermal head comprising a plurality of heater elements arranged in one direction so that fluctuation in heating temperatures of the heater elements can be suppressed and defective perforation can be prevented without deteriorating the durability of the thermal head.
In accordance with a first aspect of the present invention, there is provided a method of controlling a thermal head provided with a plurality of heater elements which are arranged in a main scanning direction, the method comprising the step of selectively energizing the heater elements according to black and white information for the pixels to be formed by the respective heater elements, wherein the improvement comprises
the step of an auxiliary heating control in which each heater element corresponding to a pixel the black and white information for which is white is heated to an auxiliary temperature according to the black and white information for pixels adjacent to the pixel in the main scanning direction.
In accordance with a second aspect of the present invention, there is provided an apparatus for controlling a thermal head provided with a plurality of heater elements which are arranged in a main scanning direction, the apparatus comprising a heater control means which selectively energizes the heater elements according to black and white information for the pixels to be formed by the respective heater elements, wherein the improvement comprises that
the heater control means is provided with an auxiliary heater control means which performs an auxiliary heating control in which each heater element corresponding to a pixel the black and white information for which is white is heated to an auxiliary temperature according to the black and white information for pixels adjacent to the pixel in the main scanning direction.
For example, when the thermal head is used in a heat-sensitive printer, the xe2x80x9cblack and white informationxe2x80x9d represents whether the pixel is to be black or white, and when the thermal head is used in a heat-sensitive plate making apparatus, the xe2x80x9cblack and white informationxe2x80x9d represents whether the pixel is to be perforated. In the former case, that the black and white information for the pixel is white means that the pixel should not be printed and in the latter case, that the black and white information for the pixel is white means that the pixel should not be perforated.
The heater control means may be further provided with a preheating heater control means which performs a preheating control in which each heater element corresponding to a pixel the black and white information for which is white is heated to a preheating temperature according to the black and white information for pixels adjacent to the pixel in a sub-scanning direction substantially perpendicular to the main scanning direction.
The auxiliary temperature or the preheating temperature means a temperature lower than a temperature at which the heater element can accomplish the expected objected, that is, a temperature at which the heater element cannot form a perforation through which the ink can pass (including the case where the plate material is not perforated at all) in the case where the thermal head is used in a heat-sensitive plate making apparatus and a temperature at which the heater element cannot form a black spot in the case where the thermal head is used in a heat-sensitive printer.
The heater control means may be further provided with a heat history heater control means which performs a heat history control in which each heater element corresponding to a pixel the black and white information for which is black is additionally heated on the basis of the black and white information for pixels adjacent to the pixel in the main scanning direction and/or a sub-scanning direction substantially perpendicular to the main scanning direction.
The auxiliary heater control means, the preheating heater control means and the heat history heater control means may be arranged to perform the heater control on the basis of the black and white information for pixels adjacent to said adjacent pixels and/or pixels positioned in an oblique direction thereof in addition to the black and white information for said adjacent pixels. Further, the auxiliary heating control and the heat history control may be performed at the same timing.
The thermal head may be used, for instance, in a heat-sensitive plate making apparatus, and the auxiliary heater control means may perform the auxiliary heating control so that each heater element corresponding to a pixel the black and white information for which is white is heated to an auxiliary temperature before heater elements corresponding to pixels the black and white information for which is black are heated to perforate the heat-sensitive plate material.
The thermal head may be arranged so that a set of heater elements which are contiguously arranged in the main scanning direction and/or the sub-scanning direction form a single pixel. In this case, the auxiliary heating control, the preheating control and the heat history control are performed on a set of heater elements corresponding to each pixel.
In accordance with the method and apparatus of the present invention, since even the heater elements which need not be heated to perforate the plate material (heater elements corresponding to white pixels: pixels the black and white information for which is white) are heated to a certain elevated temperature, the heating temperature of a heater element which corresponds to a black pixel (a pixel the black and white information for which is black) and which is adjacent to a heater element corresponding to a white pixel can be heated to a temperature close to a heating temperature of a heater element corresponding to a black pixel which is interposed between heater elements corresponding to black pixels, fluctuation in the heating temperature of the heater elements can be suppressed without deteriorating the durability of the thermal head. That is, when the thermal head is used in a heat-sensitive plate making apparatus, perforations can be uniform in size.
When the preheating control is performed in addition to the auxiliary heating control, a heater element which comes to correspond to a black pixel after a white pixel can be heated to a temperature close to a heating temperature of a heater element successively corresponding to two black pixels, whereby fluctuation in the heating temperature of the heater elements can be suppressed without deteriorating the durability of the thermal head. That is, when the thermal head is used in a heat-sensitive plate making apparatus, perforations can be further uniform in size.
When the heat history control in which each heater element corresponding to a black pixel is additionally heated on the basis of the black and white information for pixels adjacent to the pixel in the main scanning direction and/or a sub-scanning direction substantially perpendicular to the main scanning direction is performed in addition to the auxiliary heating control, a heater element which comes to correspond to a black pixel after a white pixel can be heated to a temperature close to a heating temperature of a heater element successively corresponding to two black pixels, whereby fluctuation in the heating temperature of the heater elements can be suppressed without deteriorating the durability of the thermal head. That is, when the thermal head is used in a heat-sensitive plate making apparatus, perforations can be further uniform in size.
When the auxiliary heating control and the heat history control are performed at the same timing, the time for performing the heat history control need not be reserved separately from the time for performing the auxiliary heating control and accordingly, elongation of the line cycle in the sub-scanning direction can be avoided.
When the thermal head is used in a heat-sensitive plate making apparatus and the auxiliary heater control means performs said auxiliary heating control prior to the timing at which heater elements corresponding to black pixels are heated to perforate the plate material, energy applied to the heater elements for the auxiliary heating can be efficiently used for perforation of the plate material.
Recently, there have been developed multiple-row thermal heads having heater elements arranged in a plurality of rows extending in the main scanning direction. In such a multiple-row thermal head, a plurality of contiguous heater elements correspond to one pixel though, in the conventional single-row thermal head, one heater element corresponds to one pixel. For example, in the multiple-row thermal head disclosed in our Japanese Unexamined Patent Publication No. 2000-326474, a pair of heater elements adjacent to each other in the main scanning direction are controlled according to an image data component for one pixel. Further, in the multiple-row thermal head disclosed in Japanese Unexamined Patent Publication No. 2000-238230, a plurality of heater elements adjacent to each other in the main scanning direction and the sub-scanning direction are controlled according to an image data component for one pixel.
FIGS. 22A and 22B respectively show multiple-row thermal heads, where one pixel is formed by a pair of heater elements adjacent to each other in the main scanning direction and FIG. 22C shows a conventional thermal head, where one pixel is formed by one heater element. In the conventional thermal head shown in FIG. 22C, a plurality of heater element assemblies, each comprising a heater element 84 formed on a straight lead electrode, are arranged side by side. In the multiple-row thermal head shown in FIG. 22A, a plurality of heater element assemblies, each comprising a pair of heater elements 81 formed on a lead electrode 80 on opposite sides of a central slit, are arranged side by side. In the multiple-row thermal head shown in FIG. 22B, a plurality of heater element assemblies, each comprising a pair of heater elements 83 formed on a U-shaped lead electrode 82, are arranged side by side. In the multiple-row thermal head shown in FIG. 22A, the heater elements 81 are connected in parallel and the heater element assembly is connected to a power source at the ends of the lead electrode 80 which are on the upper and lower sides of the heater elements 80. To the contrast, in the multiple-row thermal head shown in FIG. 22B, the heater elements 83 are connected in series and the heater element assembly is connected to a power source at the ends of the lead electrode 80 which are both on the upper side of the heater elements 83. Each heater element 81 or 83 are smaller in size than a heater element 84 of the conventional thermal head shown in FIG. 22C. Accordingly, when the recording medium is perforated at 300 dpi by the multiple-row thermal head shown in FIG. 22B, the diameter of each perforation is smaller than when the recording medium is perforated at 300 dpi by the conventional thermal head shown in FIG. 22C as shown in FIGS. 23A and 23B, whereby the amount of ink transferred to the printing paper can be smaller and offset can be prevented. In the case shown in FIG. 23B, the resolution in the sub-scanning direction is 600 dpi.
In such a multiple-row thermal head, there has been a problem that the heat history control on the multiple-row thermal head in order to prevent imperforation due to fluctuation in heating temperatures of the heater elements can result in oversized perforations. For example, when a square area of the plate material is to be perforated in order to print a square such as shown in FIG. 24A, imperforation can occur at an upper edge portion 86 and left and right side edge portions 87 of the square area as shown in FIG. 24B if the heater elements of the thermal head corresponding to the square area is uniformly energized as described above. When the heat history control is effected, heater elements corresponding to pixels in the upper edge portion 86 are energized for a longer time in order to first prevent occurrence of imperforation in the upper edge portion 86. Then heater elements corresponding to pixels in the left and right edge portions 87 are energized for a longer time in order to prevent occurrence of imperforation in the left and right edge portions 87. When the heater elements corresponding to pixels in the left and right edge portions 87 are energized for a longer time, the pixels formed by the outer one of the heater element pair can be successfully perforated. However, pixels formed by the inner one of the heater element pair can be oversized since the inner one of the heater element pair is heated to an excessively high temperature due to influence of increase in temperature of the surrounding heater elements and elongation in energizing time.
Even when a multiple-row thermal head where a plurality of heater elements arranged in the main scanning direction and/or the sub-scanning direction form one pixel is used, by effecting the auxiliary heating control, the heating temperature of a heater element which corresponds to a black pixel and which is adjacent to a heater element corresponding to a white pixel can be heated to a temperature close to a heating temperature of a heater element corresponding to a black pixel which is interposed between heater elements corresponding to black pixels, fluctuation in the heating temperature of the heater elements can be suppressed without deteriorating the durability of the thermal head and without generating oversized perforations. That is, when the thermal head is used in a heat-sensitive plate making apparatus, perforations can be uniform in size.